Holographic gratings are extremely attractive for fabrication of tamper-proof or secure credit cards. Other documents are protected by incorporating a holographic grating in them for other security reasons. The present apparatus and method disclose a procedure and mechanism for forming one or more gratings so mass production from a master grating is possible.
One example of a procedure for conversion of a photograph into a hologram is set forth in U.S. Pat. No. 3,832,027. This describes a procedure which converts a two dimension photograph into a hologram. It is, however, accomplished with computer data processing to convert the photographic symbol or image from the photograph into a hologram. Multiple views are aggregated to provide the ultimate representation. The present apparatus is able to form a holographic grating (a representation appearing to have depth when viewed in conventional illumination and which is described hereinafter as a hologram or holographic grating).
Background is given in an article entitled "Diffraction Gratings" at page E-29 of the 1984 Edition of the "The Optical Industry and Systems Purchasing Directory". It describes making holographic gratings by permitting two beams of coherent monochromatic light to form grating interference fringes in a photosensitive material. The interference fringes are formed where the two coherent beams come together and overlapping beams, valleys and peaks interfere with one another for periodic cancellation. This defines interference fringes, enabling marked lines on the material. A grating is normally described by the spacing of grooves in the grating, it being possible to obtain upwards of 6,000 grooves per millimeter, and the gratings can be as large as 400 by 600 millimeters. In fact, the present means and method enable fabrication of a substantially unlimited size grating. The inventive method takes advantage of the interference pattern between the pair of interfering coherent beams (from a common laser source) which enables the interference fringes to be controlled. The present approach utilizes a photosensitive layer on a substrate as an initial blank surface. The layer is aphotosensitive material, normally a photoresist used routinely for semiconductor fabrication. One approach in the procedure is to form an image on a surface, namely as an image to be converted into a hologram. The term hologram refers to the grating which serves as a reflective diffraction grating or a transparency and further contemplates an image which appears to have 3-D depth when viewed in typical ambient lighting conditions. Thus, the term "image" refers to the symbol or object which is first provided in a 2-D depiction. This 2-D image is then converted into a 3-D hologram by the present process. Sequentially, the image is first photographed or otherwise formed into a 2-D format. It is perfectly acceptable to use 2-D photographs or, alternatively, to draw or sketch the image with suitable color contrasts. It is first scaled to the desired size. A 1:1 conversion from 2-D image to hologram is an example. The scale of a 2-D image can be varied by well known photographic processes. The image is therefore scaled to the desired size and is provided with the desired color scheme. After the image has been scaled and has the desired colors in it, it is next scanned with an XY drive system which breaks the image area into a number of pixels. Pixel size depends on the resolution (or fineness) of the system. A pixel is thus defined as the smallest unit at a location on X and Y coordinates which is to be encoded for formation of a set of code words representing the pixels which make up the XY description of the image. The scanning routine in X and Y dimensions increments at a specified spacing to hereby form a pixel at every XY intersection of the coordinate system. At each pixel, a small colored dot is defined which can be expressed in terms of optical density (a scale between total black and total white with shades of grey between) and also the three primary colors of red, green and blue. Thus, an image is divided into a multitude of equal size pixels which are evaluated for optical density and these values are then converted into suitable digital words representative of the measurements. Separately, an XY coordinate drive system locates each particular pixel. Values for X and Y are assigned on operation of the drive system and such representations are likewise formed into digital words. The individual pixel is thus represented by several data words including X and Y locations, color in the form of primary color representations and optical density. This set of data associated with an individual pixel is then recorded.
At the time of retrieval of the data representative of the individual pixel, the X and Y pixel location data is used to position a photosensitive blank driven by an XY drive system. A laser beam emitted through a shutter defines the size of the pixel on the blank, and defines a reference beam which illuminates the pixel area of the blank to develop the interference fringes which have the form of rulings on a grating in the customized diffraction grating. The reference beam is broken into four equal portions by beam splitters and three of the portions are directed from different directions to the same location on the blank. The reference beam has an exposure time arbitrarily described as 99 units duration. The three beams are directed onto the target from other angles as will be described, each persisting for an interval of up to 33 units. Each of the three beams (being equal in intensity) is assigned a ratio depending on the portion of the primary colors found in the pixel. The beams are modulated and thereby form interference fringes at the pixel to shape the rulings on the gratings. In conclusion, this forms interference fringes which are specially adapted to refract in proportion the three primary colors and therefore complete the pixel. The entire image is painted on the blank as a multitude of pixels located in the XY coordinate system to preserve and transfer the image, thereby forming a hologram on the blank. After the blank has been fully exposed, it can then be used as a master for fabrication of a multitude of replicated holograms made from the photoresist blank.
The structure of the present apparatus incorporates an XY drive system for moving a photoresist blank in front of, and at a controlled distance from a laser source along with a set of mirrors and beam splitters so that four beams are directed to a common pixel location at specified angles of convergence. This forms the individual pixel in the photoresist blank at each pixel location. The system further includes an image scanner which converts any 2-D image into the data necessary to make a hologram, the system including an XY drive system, an optical system scaled to view the defined pixel area, appropriate color filters and color density measuring apparatus. The data from the multiple measurements for each pixel are converted into digital words.